1. Field of the Invention
This invention relates to a metal powder which is used for the production of structural parts having superior mechanical properties and machinability. In particular, this invention relates to iron-graphite composite powders, the process for the preparation thereof and the process for forming parts using powder metallurgy manufacturing technology therefrom.
2. Related Background Art
Conventionally, metal parts may be manufactured by casting a liquid metal or forming or machining a solid into a specific shape or form. Malleable iron is a particularly useful material for the manufacture of metal parts because of its superior machinability, toughness, ductility, corrosion resistance, strength, magnetic properties and uniformity. These properties arise from the metallographic microstructure of the iron, which comprises carbon clusters embedded in a ferrous matrix. However, malleable iron is a cast iron.
Due to the growing demand for inexpensive, light weight machine parts, powder metallurgy (P/M) manufacturing technology is replacing conventional manufacturing practices. In powder metallurgy, a raw metal powder material is press molded to produce a green compact, which is subjected to sintering. The sintered body may be further subjected to coining, forging, heat-treatment, and occasionally cutting or machining to produce the final metal product. For example, U.S. Pat. No. 5,628,045 discloses a process for forming sintered parts having an austenitic and/or bainitic matrix by selective cooling (additional heat treatment) of a sintered article. Accordingly, the raw metal powder material used in this process must possess several important properties. The raw metal material must be suitable for press molding, and therefore must possess acceptable hardness and compressibility. The hardness of a powder will have a direct effect on its compressibilityxe2x80x94a lower hardness will result in a superior compressibility. In addition, the solid metal products produced from the raw metal material should advantageously possess mechanical strength, toughness and machinability. Thus the raw metal material used to prepare these products must also possess good heat-treatment properties, e.g., sinterability and hardenability. Several workers have attempted to prepare a powder, useful in powder metallurgy manufacturing, that would also provide sintered articles having the high graphite content and microstructure of malleable iron. For example, Yang (International Conference on Powder Metallurgy and Particulate Materials, presented Jun. 1, 1998) disclosed a sintered steel prepared by graphitization of a green compact composed of a boron and sulfur containing P/M ferrous alloy. The sintered steel possessed a ferritic matrix, wherein the graphite was precipitated in the pores of the sintered article. The graphite was a so-called xe2x80x9cfree-formxe2x80x9d graphite, because the shape of the graphite produced was dependent upon the shape of the pores in which it was precipitated.
Uenosono (Proceedings of the International Conference on Powder Metallurgy and Particulate Materials, Jun. 29-Jul. 2, 1997, Chicago, Ill.) disclosed a sintered steel, similar to that of Yang, also containing boron and sulfur and having graphite deposited in the pore sites.
Shivanath (U.S. Pat. No. 5,656,787) disclosed the use of a carbon/iron blend in the formation of sintered articles. In this case, the blend is comprised of comparatively small particles of carbon dispersed within the voids formed by the comparatively larger ferro alloy particles. Ovecoglu (Intl. J. Powder Metallurgy, 1998) discloses attrition milling of iron powder and graphite powder to form iron-carbon powder composite alloys. Milling of the iron-graphite powder mixture for extended periods of time results in the gradual disappearance of graphite. X-ray diffraction spectra of the powder obtained after milling for 20 hours indicates that the powder particles contain only xcex1-Fe.
These methods, however, do not produce P/M powders which possess the metallographic microstructure or the desirable properties of malleable iron. Moreover, these methods do not provide for the efficient production of such powders in quantity. Accordingly, it would be desirable to provide an iron-graphite composite powder that provides the advantages of malleable iron and that may be used to produce sintered articles using powder metallurgy manufacturing technology.
This invention relates to a novel iron-graphite composite powder having a microstructure comprising carbon clusters in a ferrous matrix. Another embodiment of this invention relates to a process of preparing this iron-graphite composite powder comprising the steps of:
(a) atomizing a liquid iron to form an atomized iron powder;
(b) heating the atomized iron powder to a first stage graphitization temperature; and
(c) cooling the powder from the first stage graphitization temperature to a second stage graphitization temperature.
This invention also relates to a sintered article prepared by the process of sintering the iron-graphite composite powder of this invention. In another embodiment, this invention relates to sintered articles, produced from the iron-graphite composite powder of this invention, which have been subjected to post-sintering treatments.